How to Match Levels?

I want to do some testing of digital sources with my Stax Omega phones, but I have no idea how to properly match the levels to within the golden .1db goal. I have heard that a sound meter (a la Radioshack) won't cut it.. why not? If I place the phones properly and place the mic between the phones and play 1 KHZ tone or something, how close will I be getting to properly matched levels. After years of buying gear based on sound and opinions, I honestly want to test some things now, and am surprised at how little info is available...

Any help?

-Adam